Bolts and bolt systems are crucial components in modern manufacturing having a vast range of applications in both commercial and industrial products. Although seemingly straight forward in use, those skilled in the art will recognize that bolts rely on a relatively complex combination of mechanical properties and design features that enable them to function properly. More specifically, a bolt's utility is not determined simply by whether it fits into a hole or not, but rather, whether it complies with the applicable specification including physical dimensions, thread density, forming material, and tightening requirements. The complimentary linkage of nuts and bolts can also significantly affect component assemblies and product efficiency. For example, using a bolt that is too long for a given application may interfere with the proper operation of surrounding components. Similarly, mounting a bolt that has insufficient tensile strength for the specified application, or tends to loosen under vibration or relative transverse motion of the secured components, may cause equipment to prematurely wear or fail.
A bolt thread, a main characteristic of any bolt or bolt system, functions as an extension of the basic machine concept of an inclined plane wrapped around a shaft. That is, when the thread is turned, it moves the mating part or nut up the inclined plane. This relative motion between the nut and the bolt functions to reduce the distance between the bearing surfaces of the bolt and nut. This dimension is referred to as the “grip length” of the bolted joint. When more turning force or “torque” is applied to the shaft, the more force is exerted on the nut. This force creates a tension or “preload” force in the bolt, which in turn, clamps the mating parts together. Generating and maintaining sufficient preload force is the key to strong and reliable bolted joints that will not loosen or break under load. Those skilled in the art will recognize that a fastener, when tightened, functions by analogy, much like a spring where rotating the bolt stretches the spring and generates a preload force. The more the bolt is rotated (and the joint members within the grip resist), the more it stretches and generates more preload or tension and simultaneously compresses the components together. “Clamping force” is thus defined as the difference between the preload force and the tension force on the joint. Thus, where no tension loads are applied to a joint, the clamping force equals the preload force. If the tension load is equal to the preload, there is no clamping force. Accordingly, if the tension load is increased beyond the initial preload force, the joint will separate. After a joint separates, it will continue to incur increased tensile loads until the ultimate tensile strength of the fastener is reached and the fastener breaks. Thus, as a practical matter, joint failure typically occurs well before a fastener actually breaks because the parts that are being held together will loosen and not function properly. In view of the foregoing, proper preload, and thus proper clamping force, must therefore be developed and maintained in order to reduce the likelihood of a variety of joint problems such as fatigue, separation, and “self-loosening” from vibration and/or transverse motion of the involved components, any one or combination of which may lead to joint failure.
Joints are also subject to load by shear force, tension force, or a combination of both. Where a joint is loaded in tension, the preload force on the bolt functions to oppose the applicable joint separating forces. Thus, the ultimate strength of a joint is limited by the strength of the bolt creating the joint. Accordingly, the higher the preload force the better the joint as it will prevent the assembled parts from moving and the joint from loosening. Stated more simply, the preload force determines the strength of a joint. It is therefore important that preload force be maintained in a fastener during operation. Shear forces result when friction between the fastened parts inhibits movement. Thus, the friction carries all or part of the load, not the fastener. Here, the greater the preload force, the greater the clamping force, the greater the friction, and the stronger the joint. Absent other forces such as vibration, a properly designed and tightened joint will not experience a direct shear load.
In summary, depending on the design and specific application, a bolt may loosen in service (i.e. lose its tension or preload) for a variety of causes, including any or all of the following: (1) vibration, which can create relative transverse movement of the bolted materials leading to self loosening of the nut; (2) relaxation of the bolted joint after tightening, such as, for example, due to embedment or gasket creep; (3) elastic interactions when multiple bolts are present in a bolted joint; (4) temperature fluctuations of the components; (4) insufficient preload developed at installation; and (5) operation (including improper operation) of the bolted joint components.
Consider, for example, the use of component fasteners in removable accordion gates as have been used in indoor and outdoor retail and warehouse environments to temporarily prevent access to designated service and work areas. Such areas may include, for example, shopping aisles, checkout aisles and the like, as may be defined by shelving and storage racks such as pallet rack systems and assemblies. Accordion gates used in these applications typically comprise a plurality of interconnected and moveable accordion elements or components that cooperate together (i.e. rotate transversely to one another about a corresponding common axis and joint) to permit the respective gates to be unitarily extensible and retractable, as limited by the number and length of the corresponding elements. Accordion gates are typically mounted and substantially permanently affixed in the aforementioned entryways at their trailing gate ends to receiving outwardly facing pallet post sections by threading one or more bolts through corresponding receiving keyholes or apertures stamped in the respective components. When not required for use, the gates are secured to the receiving posts in substantially retracted storage positions, limited by the number and width of extensible accordion elements, using chains, flexible cord, or other suitable retention devices. When it is subsequently desirable to block and inhibit access to a corresponding entryway and/or aisle section, the retention device is removed or disabled, whereupon the gate may be extended and secured at its leading end to a corresponding opposing and receiving outwardly facing pallet post section in the same or similar manner using one or more bolts threaded through receiving keyholes or apertures.
Accordion gates thus function to expand and contract by the relative transverse movement of the joined members or “slats” about a common joint and axis. Typically these joints comprise barrel bolts (and mating screws), but may also utilize a variety of removable fasteners including conventional carriage bolts, hex bolts, flange bolts, and shoulder bolts. As those skilled in the art will recognize, a barrel bolt (also known in the art as a “sex bolt”) is a type of mating fastener combining an internally threaded (female) fastener barrel with an externally threaded (male) fastener screw. Barrel bolts are typically used for through bolting applications where a low profile bolt head is desired on both sides of a joint (i.e. both sides of the joined gate slats in a removable accordion gate).
As referenced above, depending on the design and specific application, conventional fasteners, including the above referenced barrel bolts, may lose their tension or preload in service for a variety of causes. In accordion gates and similar applications, a principal cause of such tension loss is the relative transverse motion of the cooperating slats about their common joint and axis. More specifically, the repetitive transverse motion resulting from expanding and contracting the gate (and the cooperating slats) applies opposing forces on each bolt barrel portion and each nut (fastener screw in a barrel bolt) that may result in the loosening (i.e. unscrewing) of the respective joints ultimately leading to joint fatigue or failure, which in turn, may lead to fatigue or failure of the gate itself.
To address the above issues, manufacturers have for some fastener applications, including accordion gates of the type described above, replaced conventional removable fasteners (such as the above referenced barrel bolts) with low profile permanent mechanical fasteners such as rivets. As those skilled in the art will recognize, a rivet is a permanent fastener generally comprising a smooth cylindrical shaft with a “factory head” on one end and a “tail” (also interchangeably referred to as a “shop tail” or “buck tail”) on the opposite end thereof. Upon installation, the rivet is placed in an aperture such as a punched or drilled hole and the tail is then permanently deformed about the hole to mechanically hold the fastener in place. Specifically, the tail is “upset” or “bucked”, so that it expands to a multiple of its original size (typically 1.5 times the original shaft diameter), while holding the rivet in place. The deforming action functions to create a new head on the other end by smashing the tail material flatter, resulting in a rivet having a low profile dumbbell shape. Because there is effectively a head on each end of the installed rivet, it can support both tension loads (loads parallel to the shaft) as well as shear loads (loads perpendicular to the shaft). Moreover, because the rivet is permanently installed, it generally resists joint loosening from opposing transverse motion such as found in the above referenced accordion gates. Such joints, however, are still not impervious to joint fatigue or failure. When such joints (or the affixed components) do fatigue or fail, the joint must be replaced, as rivets are generally not amenable to repair. In such case, each joint must be drilled through to physically remove the rivet material. This activity is typically performed off-site requiring substantial time, labor, and expense, and resulting in loss of use by the customer of the product itself. In the case of accordion gates described above, such removal and repair may be problematic for the customer as an entryway, aisle, or other designated area, may be left open or require installation of a new or temporary gate during the off-site repair of the damaged gate resulting in additional time and expense for the customer and/or the service technician.
Accordingly, there is a need for an improved fastener, and more specifically, an improved barrel bolt fastener assembly, that overcomes the disadvantages of the prior art by inhibiting or preventing the loss of joint tension and preload (including, but not limited to, such loss or loosening resulting from relative transverse motion of the clamped components). Still further, there is a need for such an improved barrel bolt fastener assembly that functions to inhibit and prevent the aforementioned loss of joint tension and preload as may occur, for example, but not limitation, in the exemplary removable accordion gate application above, thereby inhibiting or preventing fatigue and failure of the component joints as well as the applicable gate.